Cenozoic Paleoclimate on Land in North America

Paleotemperature and paleoprecipitation over the past 40 m.yr. can be inferred from the degree of chemical weathering and depth of carbonate nodules in paleosols of Oregon, Montana, and Nebraska. Paleosol records show that late Eocene (35 Ma), middle Miocene (16 Ma), late Miocene (7 Ma), and early Pliocene (4 Ma) warm climatic episodes were also times of a wet climate in Oregon, Montana, and Nebraska. Oregon and Nebraska were humid during warm‐wet times, but Montana was no wetter than subhumid within the rain shadow of intermontane basins. Global warm‐wet paleoclimatic spikes steepened rather than flattened geographic gradients of Rocky Mountain rain shadows. Long‐lived mountain barriers created dusty dry basins with sedimentation rates high enough to preserve Milankovitch‐scale (100–41 kyr) global paleoclimatic variation in some sequences of paleosols. Greenhouse warm‐wet climates indicated by paleosols were also peaks of diversity for North American plants and animals and coincided with advances in coevolution of grasses and grazers. Paleosol records differ from global compilations of marine foraminiferal oxygen and carbon isotopic composition, due to competing influences of global ice volume and C4 grass expansion. Paleosol records support links between global warming and high atmospheric CO2.